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Re: [Qemu-devel] [PATCH v2] block: fix QEMU crash with scsi-hd and drive


From: Greg KH
Subject: Re: [Qemu-devel] [PATCH v2] block: fix QEMU crash with scsi-hd and drive_del
Date: Thu, 24 May 2018 17:02:12 +0200
User-agent: Mutt/1.10.0 (2018-05-17)

On Thu, May 24, 2018 at 11:09:47AM +0200, Greg Kurz wrote:
> Removing a drive with drive_del while it is being used to run an I/O
> intensive workload can cause QEMU to crash.
> 
> An AIO flush can yield at some point:
> 
> blk_aio_flush_entry()
>  blk_co_flush(blk)
>   bdrv_co_flush(blk->root->bs)
>    ...
>     qemu_coroutine_yield()
> 
> and let the HMP command to run, free blk->root and give control
> back to the AIO flush:
> 
>     hmp_drive_del()
>      blk_remove_bs()
>       bdrv_root_unref_child(blk->root)
>        child_bs = blk->root->bs
>        bdrv_detach_child(blk->root)
>         bdrv_replace_child(blk->root, NULL)
>          blk->root->bs = NULL
>         g_free(blk->root) <============== blk->root becomes stale
>        bdrv_unref(child_bs)
>         bdrv_delete(child_bs)
>          bdrv_close()
>           bdrv_drained_begin()
>            bdrv_do_drained_begin()
>             bdrv_drain_recurse()
>              aio_poll()
>               ...
>               qemu_coroutine_switch()
> 
> and the AIO flush completion ends up dereferencing blk->root:
> 
>   blk_aio_complete()
>    scsi_aio_complete()
>     blk_get_aio_context(blk)
>      bs = blk_bs(blk)
>  ie, bs = blk->root ? blk->root->bs : NULL
>             ^^^^^
>             stale
> 
> The problem is that we should avoid making block driver graph
> changes while we have in-flight requests. This patch hence adds
> a drained section to bdrv_detach_child(), so that we're sure
> all requests have been drained when blk->root is freed.
> 
> Signed-off-by: Greg Kurz <address@hidden>
> ---
> v2: - drain I/O requests when detaching the BDS (Stefan, Paolo)
> ---
>  block.c |    4 ++++
>  1 file changed, 4 insertions(+)

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>



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